Spherical gridding in tension of data on a sphere
gmt sphinterpolate [ table ] -Ggrdfile [ -Iincrement ] [ -Qmode[options] ] [ -Rregion ] [ -V[level] ] [ -Z ] [ -bibinary ] [ -dinodata ] [ -eregexp ] [ -hheaders ] [ -iflags ] [ -rreg ] [ -:[i|o] ] [ --PAR=value ]
Note: No space is allowed between the option flag and the associated arguments.
sphinterpolate reads one or more ASCII [or binary] files (or standard input) containing lon, lat, z and performs a Delaunay triangulation to set up a spherical interpolation in tension. The final grid is saved to the specified file. Several options may be used to affect the outcome, such as choosing local versus global gradient estimation or optimize the tension selection to satisfy one of four criteria.
- Name of the output grid to hold the interpolation.
- One or more ASCII (or binary, see -bi[ncols][type]) data table file(s) holding a number of data columns. If no tables are given then we read from standard input.
- x_inc [and optionally y_inc] is the grid spacing. Optionally, append a suffix modifier. Geographical (degrees) coordinates: Append m to indicate arc minutes or s to indicate arc seconds. If one of the units e, f, k, M, n or u is appended instead, the increment is assumed to be given in meter, foot, km, Mile, nautical mile or US survey foot, respectively, and will be converted to the equivalent degrees longitude at the middle latitude of the region (the conversion depends on PROJ_ELLIPSOID). If y_inc is given but set to 0 it will be reset equal to x_inc; otherwise it will be converted to degrees latitude. All coordinates: If +e is appended then the corresponding max x (east) or y (north) may be slightly adjusted to fit exactly the given increment [by default the increment may be adjusted slightly to fit the given domain]. Finally, instead of giving an increment you may specify the number of nodes desired by appending +n to the supplied integer argument; the increment is then recalculated from the number of nodes and the domain. The resulting increment value depends on whether you have selected a gridline-registered or pixel-registered grid; see GMT File Formats for details. Note: if -Rgrdfile is used then the grid spacing has already been initialized; use -I to override the values.
- Specify one of four ways to calculate tension factors to preserve local shape properties or satisfy arc constraints [Default is no tension].
- Use piecewise linear interpolation; no tension is applied.
- Smooth interpolation with local gradient estimates.
- Smooth interpolation with global gradient estimates. You may optionally append N/M/U, where N is the number of iterations used to converge at solutions for gradients when variable tensions are selected (e.g., -T only) , M is the number of Gauss-Seidel iterations used when determining the global gradients , and U is the maximum change in a gradient at the last iteration [0.01].
- Use smoothing. Optionally append E/U/N [/0/0/3], where E is Expected squared error in a typical (scaled) data value, and U is Upper bound on weighted sum of squares of deviations from data. Here, N is the number of iterations used to converge at solutions for gradients when variable tensions are selected (e.g., -T only) 
- west, east, south, and north specify the region of interest, and you may specify them in decimal degrees or in [±]dd:mm[:ss.xxx][W|E|S|N] format Append +r if lower left and upper right map coordinates are given instead of w/e/s/n. The two shorthands -Rg and -Rd stand for global domain (0/360 and -180/+180 in longitude respectively, with -90/+90 in latitude). Set geographic regions by specifying ISO country codes from the Digital Chart of the World using -Rcode1,code2,…[+r|R[incs]] instead: Append one or more comma-separated countries using the 2-character ISO 3166-1 alpha-2 convention. To select a state of a country (if available), append .state, e.g, US.TX for Texas. To specify a whole continent, prepend = to any of the continent codes AF (Africa), AN (Antarctica), AS (Asia), EU (Europe), OC (Oceania), NA (North America), or SA (South America). Use +r to modify the bounding box coordinates from the polygon(s): Append inc, xinc/yinc, or winc/einc/sinc/ninc to adjust the region to be a multiple of these steps [no adjustment]. Alternatively, use +R to extend the region outward by adding these increments instead [no extension]. Alternatively for grid creation, give Rcodelon/lat/nx/ny, where code is a 2-character combination of L, C, R (for left, center, or right) and T, M, B for top, middle, or bottom. e.g., BL for lower left. This indicates which point on a rectangular region the lon/lat coordinate refers to, and the grid dimensions nx and ny with grid spacings via -I is used to create the corresponding region. Alternatively, specify the name of an existing grid file and the -R settings (and grid spacing, if applicable) are copied from the grid. Appending +uunit expects projected (Cartesian) coordinates compatible with chosen -J and we inversely project to determine actual rectangular geographic region. For perspective view (-p), optionally append /zmin/zmax. In case of perspective view (-p), a z-range (zmin, zmax) can be appended to indicate the third dimension. This needs to be done only when using the -Jz option, not when using only the -p option. In the latter case a perspective view of the plane is plotted, with no third dimension.
- Use variable tension (ignored with -Q0 [constant]
- -V[level] (more …)
- Select verbosity level [c].
- Before interpolation, scale data by the maximum data range [no scaling].
- -bi[ncols][t] (more …)
- Select native binary format for primary input. [Default is 3 input columns].
- -dinodata (more …)
- Replace input columns that equal nodata with NaN.
- -e[~]”pattern” | -e[~]/regexp/[i] (more …)
- Only accept data records that match the given pattern.
- -h[i|o][n][+c][+d][+rremark][+rtitle] (more …)
- Skip or produce header record(s).
- -:[i|o] (more …)
- Swap 1st and 2nd column on input and/or output.
- -r (more …)
- Set node registration [gridline].
- -^ or just -
- Print a short message about the syntax of the command, then exits (NOTE: on Windows just use -).
- -+ or just +
- Print an extensive usage (help) message, including the explanation of any module-specific option (but not the GMT common options), then exits.
- -? or no arguments
- Print a complete usage (help) message, including the explanation of all options, then exits.
- Temporarily override a GMT default setting; repeatable. See gmt.conf for parameters.
ASCII Format Precision¶
The ASCII output formats of numerical data are controlled by parameters in your gmt.conf file. Longitude and latitude are formatted according to FORMAT_GEO_OUT, absolute time is under the control of FORMAT_DATE_OUT and FORMAT_CLOCK_OUT, whereas general floating point values are formatted according to FORMAT_FLOAT_OUT. Be aware that the format in effect can lead to loss of precision in ASCII output, which can lead to various problems downstream. If you find the output is not written with enough precision, consider switching to binary output (-bo if available) or specify more decimals using the FORMAT_FLOAT_OUT setting.
To interpolate the points in the file testdata.txt on a global 1x1 degree grid with no tension, use
sphinterpolate testdata.txt -Rg -I1 -Gsolution.nc
Renka, R, J., 1997, Algorithm 772: STRIPACK: Delaunay Triangulation and Voronoi Diagram on the Surface of a Sphere, AMC Trans. Math. Software, 23(3), 416-434.
Renka, R, J,, 1997, Algorithm 773: SSRFPACK: Interpolation of scattered data on the Surface of a Sphere with a surface under tension, AMC Trans. Math. Software, 23(3), 435-442.